1. Field of the Invention
The present invention relates to a microelectromechanical system (MEMS); and, more particularly, to a method for manufacturing a floating structure of a MEMS.
This work was supported by the IT R&D program of MIC/ IITA [2006-S-007-01, “Ubiquitous Health Monitoring Module and System Development”].
2. Description of Related Art
In general, a MEMS manufacturing technology is based on a semiconductor manufacturing technology. Well-know MEMS manufacturing technologies include a surface micromachining MEMS manufacturing technology, and a bulk micromachining MEMS manufacturing technology. In those technologies, a process of removing a sacrificial oxide layer is commonly used to float a structure of a specific part. Also, when a structure with a deep trench is formed, an anisotropic wet etching process used in the bulk micromachining MEMS manufacturing technology is generally used.
As one of methods of floating a structure by using a sacrificial oxide layer, a sacrificial layer pattern under a structure to be floated is separately defined and then etched to float the structure. As another method, isotropic etching is performed through an etching window by using a phosphosilicate glass layer as a sacrificial layer to float a polysilicon structure. The phosphosilicate glass layer has a higher etch rate than a silicon oxide (SiO2) layer, and is formed using low-pressure chemical vapor deposition (LPCVD).
As mentioned above, in the methods of floating a structure using a sacrificial oxide layer, a polysilicon pattern is formed on a silicon oxide layer used as a sacrificial layer, and then the silicon oxide layer, i.e., the sacrificial layer is removed by using a fluoric acid (HF)-based etching solution having high selectivity between the silicon oxide layer and the polysilicon pattern, thereby floating the structure formed of the polysilicon pattern or ensuring a required space under the polysilicon pattern.
Besides a structure partially floated in the sacrificial layer etching process, the sacrificial layer under the floating structure, which serves as a floating structure anchor part is also etched simultaneously at the same rate. Accordingly, the size of the sacrificial layer corresponding to the floating-structure anchor portion is designed to be greater than a pattern area of the structure to be floated, so that the floating structure is mounted on a substrate.
However, in the conventional method of manufacturing a floating structure, a surface of a doped polysilicon layer is damaged while the sacrificial layer is etched by the HF-based etching solution. Particularly, to form a polysilicon structure with a large area, the sacrificial layer is etched such that side etching is performed symmetrically with respect to an etching start position.
For this reason, if process time for etching the sacrificial layer increases, a portion of the polysilicon layer close to the etching start position contacts the HF-based etching solution for a longer period of time, as compared to a portion of the polysilicon layer at an etching end position. This damages the thin doped-polysilicon structure.
To manufacture a floating structure with a deep trench, an anisotropic wet etching process is generally performed to form a trench in a substrate. In this case, the trench forming process may take a long time because of variations in etching rate of a silicon substrate. That is, the long-time trench forming process is caused because an etching rate is significantly small at a side portion of the silicon substrate as compared to an etching rate at another portion of the silicon substrate in a depth direction.
Therefore, an etching process must be performed for a sufficiently long time to form a trench with a desired depth and width, which lengthens process time.